1. Field of the Invention
The present invention relates to a method for connecting an insulator coated wire with a conductive member by means of resistance welding.
2. Description of Related Art
FIGS. 14 and 15 shows methods for connecting an insulator coated wire with a conductive member. In the method shown in FIG. 15, an insulator coating film 915 is removed from a joint section 911 of the wire 91 by a mechanical or chemical method and then solder 93 is applied to the joint section 911 to connect with the conductive member 92. In the method shown in FIG. 14, the joint section 911 is connected with the conductive member 92 by means of resistance welding after removing the insulator coating film 915. There are also shown welding electrodes 951 and 952 and a power transformer 955 in FIG. 14. Jet 930 of the solder is schematically shown in FIG. 15.
For the case when the heat resistance of the insulator coating film 915 is low (i.e. when the coating film is made of polyurethane, polyester and the like), there has been known a method of melting the insulator coating film 915 by the solder 93, without removing the insulator coating film 915, and of soldering the insulator coated wire 91 with the conductive member 92 as shown in FIG. 16.
Similarly to that, there has been known a method of melting the insulator coating film 915 by heat, without removing it, and of directly resistance-welding the insulator coated wire 91 with the conductive member 92. In this method, for example, the insulator coated wire 91 is pinched between the conductive member 920 which is bent in the shape of a letter U. The insulator coated wire 91 and the conductive member 920 are pressed with force F applied by electrodes 951 and 952 as shown in FIGS. 17A and 17B. In this state, a current I is fed between both electrodes 951 and 952. Then, the insulator coating film 915 melts by the Joule heat generated by the current I, thus resistance-welding the insulator coated wire 91 and the conductive member 920. Such a method of jointing the insulator coated wire and the conductive member without removing the insulator coating film 915 of the insulator coated wire 91 is very efficient.
However, there has been a problem in the method of melting the insulator coating film by the heat of the solder that the insulator coating film may not melt fully when the melting point of the insulator coating film is high. As a result, the conduction resistance of the joint section between the insulator coated wire and the conductive member may become large.
According to the resistance welding method of melting the insulator coating film with heat generated by the current as described above, there has been no problem in melting the insulator coating film because the temperature becomes high. However, there has been another problem that when the diameter of the insulator coated wire is small, it is difficult to attain both objects of keeping the conduction resistance fully low and of obtaining enough joint strength at the joint section of the insulator coated wire with the conductive member. That is, the joint section must be heated up fully in order to lower the conduction resistance of the joint section by removing tidily the coating film, i.e., an insulator. However, when the insulator coated wire is pressed under the high temperature, it deforms significantly and its strength is weakened. That is, a wire whose sectional profile has been circular (diameter: D0) before welding is deformed into the oval shape (minor axis: D2) in the typical case as shown in FIG. 12C for example. As a result, the strength of the wire (tensile strength) becomes weak. When such a wire is heated to a high temperature, it gets out of shape and its shape varies, not actually assuming the fine oval shape as shown in FIG. 12C. In addition to that, its sectional area is reduced as it is extended in the longitudinal direction of the wire. As a result, the tensile strength of the joint section varies, thereby decreasing the production yield.
When the quantity of heat applied to the insulator coated wire is suppressed to avoid the above-mentioned problems, the deformation of the wire is reduced and the variation of the shape is reduced as shown in FIG. 12B for example. Therefore, enough strength may be obtained in terms of the tensile strength of the wire. However, there has been the problem that the coating film, i.e., the insulator, is not removed completely and remains at the joint section, thus increasing the electrical resistance of the joint section.
When electric power is fed to the U-shaped conductive member 920, the current I flows concentratedly through an R section 922 and heat is generated locally at this part as shown in FIGS. 17A and 17B. The temperature of the insulator coating film rises, thus melting the film, due to the heat transmitted from the R section 922. Further, the coating film melted is excreted by the pressure applied through the electrodes 951 and 952. In such a process, the higher the heat resistant temperature of the insulator coating film due to the diameter of the insulator coated wire 91 being thick for example, the more the quantity of heat required in order to melt it. It is then necessary to feed more current to the conductive member 920 in order to maintain such a quantity of heat. Due to that, it becomes necessary to increase the sectional area of the conductive member 920 so that it can withstand such a large current. Empirically, the conductive member is required to have a thickness which is equal to or larger than the diameter of the insulator coated wire 1 to be connected and a width more than twice of the diameter thereof.
Accordingly, when a conductive member having no such sectional area corresponding to the insulator coated wire to be connected is used, the current necessary for the connection cannot be fed through the conductive member, and therefore, the insulator coating film cannot be removed completely. When an excessive amount of large current is fed through the conductive member on the other hand, the R section 922 of the conductive member 920 may generate heat excessively. As a result, the temperature of the conductive member exceeds its melting temperature with the result that it melts and deforms or it is softened so that buckling thereof occurs.
Therefore, it has been essential to select the sectional area (thickness and width of the plate) of the conductive member in correspondence to the diameter of the insulator coated wire and it has been very difficult to miniaturize and to unify the conductive member in the prior art connecting method for the insulator coated wire using the conductive member 920 having a U-shaped section.
Accordingly, it is a primary object of the present invention to provide a structure and method for connecting an insulator coated wire with a conductive member which allows retaining the electrical resistance low and obtaining enough joint strength at a joint section therebetween when the insulator coated wire is jointed with the conductive member without removing the coating film of the insulator coated wire beforehand.
A secondary object of the present invention is to provide a structure and method for connecting an insulator coated wire with a conductive member which allows the conductive member to be miniaturized more than conventional ones.